1. Field of the Invention
The present invention relates to a signal amplification apparatus with advanced linearization, and more particularly, to an apparatus for removing distortion signals generated due to non-linearization of an active element when an input signal is amplified and output.
2. Description of the Related Art
In general, mobile communication systems require an apparatus for transmitting and receiving signals. When signals are transmitted from a transmission terminal to a receiving terminal, a loss occurs due to characteristics of a channel according to a transmission distance. Thus, in order to compensate for the loss, mobile communication systems require a signal amplifier.
FIG. 1 is a circuit diagram of a conventional signal amplifier. Referring to FIG. 1, the conventional signal amplifier is a common source amplifier and includes a driving unit 110 that amplifies an input signal and a resistor 120 that is a passive element. The driving unit 110 is an active element and is a three-terminal transistor including a source, a drain, and a gate. A current that flows between the drain and the source is varied by a radio frequency (RF) signal that is input to the gate. Also, the resistor 120 that is the passive element performs the function of determining a gain of the signal amplifier and an operating point of the driving unit 110.
FIG. 2 is a circuit diagram of a conventional cascode amplifier including a common source transistor and a common gate transistor. Referring to FIG. 2, the conventional cascode amplifier includes a driving unit 210 that includes a common source and a common gate in order to enhance the degree of separation of input and output, and a load terminal 220. Also, an inductor for obtaining a high gain from a high-frequency amplifier is used in the load terminal 220.
A heterodyne method that is widely used in communication systems uses an intermediate frequency so as to enhance the degree of frequency selection at a receiving side. The heterodyne method has a limitation in realizing a low cost, low power, and small RF front end. In a direct conversion method that has been suggested to solve the problems, an intermediate frequency terminal is removed to simplify processing of an RF signal. In the direct conversion method, an analog block of the communication system is minimized to realize a low cost, low power, and small mobile communication terminal. However, in the direct conversion method, inter-modulation distortion occurs due to non-linearity that occurs in each circuit.
In inter-modulation distortion, undesired frequency components are generated due to non-linear components that are generated by the active element. Thus, a transceiver for transmitting and receiving RF signals, when the gain of an input signal is high, non-linear components are relatively increased, and distortion of a transmitted signal occurs, and a problem occurs in data transmission. In digital mobile communication such as code division multiple access (CDMA), each of signals is not used in different frequency domains, and signals each having a wide frequency band are encrypted as codes and coexist in the same frequency band, and thus linearity of signals is an important factor.
A back-off method in which a laser diode having a better performance than a performance to be used is used, a feed-forward method in which only distortion components of a laser diode are extracted to generate signals having the same magnitude as that of distortion signals of a main signal and having an opposite phase to that of the distortion signals and to remove only the distortion signals, and a pre-distortion method in which signals having the same magnitude of that of distortion signals of a main signal and having an opposite phase to that of the distortion signals are generated to remove the distortion signals, have been suggested as conventional signal linearization methods. In these signal linearization methods, many RF elements are used to cause a high cost and high power consumption and the volume of a circuit is increased.
Meanwhile, in the conventional cascode amplifier of FIG. 2, a second-order derivative gm″ that is generated in Equation of a drain current IDS is minimized to remove non-linear components. However, the signal linearization method can be used only in a narrow bandwidth and is not appropriate for compensating for linearity of large signals output from a transmission unit.